1. Field of the Invention
The present invention generally relates to CCD (charge-coupled device) image sensing devices and, more particularly, is directed to an amplifying type CCD or solid state image sensing device in which an amplifying element is provided at every pixel (picture element).
2 . Description of the Related Art
Of solid state image sensors, particularly in CCD solid state image sensing devices, a signal charge stored in a photoelectric converting section of each pixel in response to an incident light is transferred by using the CCD to an output section in the form of electric charge. It is frequently observed that, during the time that the electric charges are transferred by the CCD, a noise component is mixed into the signal charges, thus resulting in the signal-to-noise ratio (S/N ratio) being deteriorated.
As a CCD solid state image sensing device proposed in order to remove the aforesaid disadvantage, an amplifying type CCD solid state image sensing device is known, in which a photosensor section comprised of a photoelectric converting section for storing a signal charge in response to an incident light, an amplifying MOS (metal oxide semiconductor) transistor for amplifying the signal charges stored in the photoelectroc converting section to output the signal charge thus amplified to a vertical signal line and reset means for resetting the input of the amplifying MOS transistor is provided at every pixel of a plurality of pixels arrayed in a two-dimensional fashion (e.g., see Japanese Laid-Open Patent Publication No. 1-154678). In this amplifying type solid state image sensing device, the amplifying MOS transistor is provided at every pixel and one load MOS transistor is connected to each vertical signal line.
The vertical signal lines are constructed by the pattern wiring process of aluminum wire or the like. In order to increase the aperture ratio of the image sensing device, it is desired that this vertical signal line is formed as thin as possible. However, if the vertical signal line is too thin, then a voltage drop occurs within the vertical signal line during the time that a current flows in the vertical signal line, resulting in a voltage difference being produced in pixels close to and distant from the load MOS transistor even though the output voltages of the pixels are the same.
A line resistance R becomes about 450 .OMEGA. at respective ends of, for example, an aluminum wire having a width of 0.8 .mu.m, a thickness of 0.4 .mu.m and a full length over a pixel region of 4 mm. At that time, insofar as the load MOS transistor has a good constant current characteristic, a current within the pixel is constant and therefore the gain is not changed between the pixels. In order to improve the constant current characteristic, a channel length L is increased and (Vg-Vth) must be reduced where Vg is the gate voltage and Vth is the threshold voltage. In that event, a current I.sub.L is expressed as: EQU I.sub.L =1/2 .mu..sub.n C.sub.ox (Vg-Vth-Vs).sup.2
where .mu..sub.n is the mobility of electron, C.sub.ox is the capacitance per unit area of an oxide film and Vs is the source voltage. There is then the problem such that, if (Vg-Vth) is small when Vs=0, then a fluctuated component of the threshold voltage Vth is squared so that the current is changed considerably.